Article pubs.acs.org/Langmuir
Direct Observation of Asphaltene Nanoparticles on Model Mineral Substrates Gijo Raj, Alain Lesimple, Jamie Whelan, and Panče Naumov* New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates S Supporting Information *
ABSTRACT: The propensity for adherence to solid surfaces of asphaltenes, a complex solubility class of heteropolycyclic aromatic compounds from the heavy fraction of crude oil, has long been the root cause of scale deposition and remains an intractable problem in the petroleum industry. Although the adhesion is essential to understanding the process of asphaltene deposition, the relationship between the conformation of asphaltene molecules on mineral substrates and its impact on adhesion and mechanical properties of the deposits is not completely understood. To rationalize the primary processes in the process of organic scale deposition, here we use atomic force microscopy (AFM) to visualize the morphology of petroleum asphaltenes deposited on model mineral substrates. High imaging contrast was achieved by the differential adhesion of the tip between asphaltenes and the mineral substrate. While asphaltenes form smooth continuous films on all substrates at higher concentrations, they deposit as individual nanoparticles at lower concentrations. The size, shape, and spatial distribution of the nanoaggregates are strongly affected by the nature of the substrate; while uniformly distributed spherical particles are formed on highly polar and hydrophilic substrates (mica), irregular islands and thicker patches are observed with substrates of lower polarity (silica and calcite). Asphaltene nanoparticles flatten when adsorbed on highly oriented pyrolytic graphite due to π−π interactions with the polycyclic core. Force−distance profiles provide direct evidence of the conformational changes of asphaltene molecules on hydrophilic/ hydrophobic substrates that result in dramatic changes in adhesion and mechanical properties of asphaltene deposits. Such an understanding of the nature of adhesion and mechanical properties tuned by surface properties, on the level of asphaltene nanoaggregates, would contribute to the design of efficient asphaltene inhibitors for preventing asphaltene fouling on targeted surfaces. Unlike flat surfaces, the AFM phase contrast images of defected calcite surfaces show that asphaltenes form continuous deposits to fill the recesses, and this process could trigger the onset for asphaltene deposition. have been advanced to explain the process of aggregation,14−17 an important understanding of the conformation of asphaltene molecules on substrates of different polarities and its impact on the adhesion property of asphaltenes is lacking. To date, most studies have investigated the mechanism of asphaltene adsorption from organic solvents on either hydrophilic clay minerals such as mica,18−21 or on minerals such as silica,22,23 calcite,24 or calcium fluoride.25 However, it is practically impossible to compare studies given the huge variety of possible asphaltene structures from different parts of the world. It is thus imperative to compare the adsorption of asphaltenes from the same reservoir source on different mineral substrates for a truly systematic understanding of the effect of surface properties in tuning the spatial distribution, conformation, and adhesion properties of asphaltene molecules at the solid−liquid interface; only then can long-term structural/
1. INTRODUCTION Oil present in reservoirs consists of many different fractions, the heaviest and most polar of which are termed “asphaltenes”.1−5 This intractable solubility class of thousands of compounds can form aggregates during the process of oil extraction, aggravated through enhanced oil recovery (EOR) processes.6 Precipitation and adhesion of these aggregates to the pipeline interior results in production blockage, oftentimes causing costly well shutdown.7−9 As oil fields mature, and EOR methods are set to become more commonplace in accessing unconventional petroleum sources,6 the associated risks of asphaltene aggregation are likely to be exacerbated, resulting in possible formation damage and restriction of oil production.10 The Yen−Mullins model11 gives a general description of the structure of asphaltenes as a class of compounds consisting of a polycyclic aromatic core with appended alkyl side chains, although their exact chemical structure and composition vary depending on the geographical location of the oil reservoir. While there have been recent advances in the structural elucidation of individual asphaltene molecules12,13 and models © 2017 American Chemical Society
Received: March 14, 2017 Revised: May 7, 2017 Published: May 29, 2017 6248
DOI: 10.1021/acs.langmuir.7b00866 Langmuir 2017, 33, 6248−6257
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Langmuir
concentration of the toluene solution using a rotary evaporator, the asphaltenes were obtained as black solid material. 2.2. Atomic Force Microscopy (AFM). The AFM measurements were performed using a commercial atomic force microscope (Model 5500, Keysight Technologies, USA) under ambient conditions (55% relative humidity (RH) and 22 °C). Images were recorded in tapping mode using standard short cantilevers (Model PPP-NCHR, Nanosensors, resonance frequency 330 kHz, spring constant 42 N m−1). The nominal radius of the tip was
